The present invention relates to a method of controlling a change of ratio in a transmission device, in particular an automatic multiple-ratio transmission device.
The present invention also relates to a transmission device implementing such a method.
From WO-A-9207206 there is known an automatic transmission in which a clutch selectively connects two rotary members of a differential gearing, such as an epicyclic train, according to whether one or the other of two antagonistic forces dominates. The first force is for example a gearing reaction, more particularly an axial thrust produced by a pinion with helical teeth mounted in an axially mobile manner, tending to disengage the clutch. The second force, tending to engage the clutch, can be produced by springs and/or by a centrifugal tachometric means. When the clutch is disengaged, it is necessary to prevent the rotation of a third rotary member of the differential gearing, and this can be ensured by a free wheel preventing this third member from rotating in the reverse direction.
This type of transmission is very advantageous as its basic functioning requires neither an external power source, nor sensors, nor a control circuit. It is the transmission device itself which produces the forces which will control it and these forces are at the same time a measurement of the parameters necessary for the control.
For modern transmissions having to provide a high level of comfort and of optimisation of the operation, the previously mentioned forces are advantageously completed by additional forces, for example produced by hydraulic actuators. The additional forces can be used to modify at will the speed and torque conditions under which the transmission changes ratio, or for locking the transmission in a given ratio when this is required (PCT/FR 94/00 176).
From another aspect, it has been observed, according to the invention, that the change of ratio under the action of forces such as a centrifugal force or a gearing reaction could exhibit certain defects, such as excessive slowness.
Furthermore, when the transmission offers a number of transmission ratios which is high in comparison with the number of gear trains used, there is in general at least one ratio change process which necessitates releasing one coupling and activating another, whilst synchronising these two operations perfectly. Any imperfection in this synchronization makes the change of ratio uncomfortable for the occupants of the vehicle and introduces stresses and/or shocks, which generate wear, in the transmission.
According to US-A-4 713 984, the coupling which must be activated receives an engaging force which is at first small and then increases progressively up to the maximum value, whilst the engaging force of the other coupling is progressively released. The hydraulic means for implementing this solution are complex, expensive and difficult to perfect.
The DE-A-41 19 078 teaches to adjust the hydraulic pressure which is used for performing a ratio change in an automatic transmission, this adjustment being made as a function of the position of the gas throttle of the engine. The DE-A-41 19 078 furthermore teaches to modify the hydraulic pressure only after a certain delay in case of a variation of the position of the gas throttle, thereby to take into account the delay of variation of the engine torque as a result of the new position of the throttle. The tuning of such a system to every possible practical situation is extremely complicated, and abrupt shocks or on the contrary excessive slippings cannot be avoided with certainty. The tuning of the system depends upon the correct operation of the engine and regresses as the engine and the transmission device wear.
The purpose of the present invention is to provide better control of the ratio change process involving the actuating of at least one selective coupling means.
According to the invention, the method for controlling progressive change from an old transmission ratio to a new transmission ratio in a transmission device offering at least two transmission ratios and comprising an actuator actuating a selective coupling means of the transmission device, wherein after initiation of the ratio change process, there is detected at least one operation-related physical magnitude and the actuator is controlled according to the detected value of the physical magnitude, is characterised in that the physical magnitude which is detected is a physical magnitude which is influenced by the progressive transmission ratio change process.
Instead of taking into account parameters which influence the behaviour of the transmission, such as the load of the engine, and the influence of which has to be predicted for trying to compensate the effect thereof on the quality of the ratio change, as is made according to DE 41 19 078, the invention takes advantage from a physical magnitude which on the contrary is influenced by the ratio change which is in progress. The actuator is accordingly controlled on the basis of the actually detected effects, instead of on the basis of the presumable effects of a circumstantial parameter.
As explained in WO-A-92 07 206, the initiation of the ratio change process can be due to a spontaneous slipping of the coupling means when the torque to be transmitted exceeds the transmission capability of the coupling means subjected to a well-determined engagement force which can for example be constant or increasing with a speed of rotation.
The invention applies in particular to the case in which two selective coupling means must change state in a synchronized manner. The initiation of the ratio change process can therefore be performed by the second selective coupling means, that is to say the selective coupling means other than the one which is controlled as a function of the physical magnitude. It is then advantageous to choose as the second selective coupling means the one whose actuation causes the input speed of transmission device to vary in the sense corresponding to the ratio change to be effected. For example, if the ratio change in the process of being carried out is a change to a higher ratio, which will therefore result in a reduction of the input speed of rotation of the transmission, action is carried out such that the ratio change process begins by actuating that one of the two coupling means which causes a reduction in the input speed of the transmission. When the detected physical magnitude reaches a certain predetermined value, the actuation of the other coupling means is initiated in its turn.
As a physical magnitude characteristic of the evolution of the ratio change process, it is, more particularly in light of the foregoing considerations, advantageous to choose the input speed, or the ratio between the input speed and the output speed of the transmission, or the ratio between a speed measured upstream and another one measured downstream of the one or more coupling means whose state changes when the ratio change process is initiated.
Preferably, the coupling means are controlled on the basis of two different truth tables. When the physical magnitude passes a predetermined threshold, there is a change from the first of the truth tables, which does not provide for the actuation of the first coupling means for the vehicle""s operating conditions which prevail at the time of the ratio change, to the second of the said tables, which provides for the actuation of the first coupling means for the said conditions.
According to a second aspect of the invention, there is proposed a transmission device for a vehicle comprising at least one gear train and able to change from an old transmission ratio to a new transmission ratio by actuating a selective coupling means, the device comprising
initiating means to initiate a change of transmission ratio as a function of at least one operating parameter of the vehicle;
means of detecting a physical magnitude of operation; and
control means for controlling the actuation of the coupling means according to the evolution of the value of the said physical magnitude,
characterized in that the detection means are designed for detecting a physical magnitude which is capable of being influenced by the progressive change of transmission ratio after its initiation.
In the rest of the description, a transmission ratio will be referred to as xe2x80x9clowxe2x80x9d when it corresponds to a low speed at the output with respect to the input speed. In the opposite case, the ratio is referred to as xe2x80x9chighxe2x80x9d.
Other features and advantages of the invention will furthermore emerge from the following description, given with reference to non-limitative examples.
In the accompanying drawings:
FIG. 1 is a diagrammatic half-view in longitudinal cross-section of a two-ratio transmission device according to the invention, in the rest state;
FIGS. 2 and 3 are views similar to FIG. 1 but relating to the operation as a reduction gear and as a direct drive respectively;
FIG. 4 is a diagrammatic half-view of a four-ratio transmission device according to the invention;
FIG. 5 shows a flowchart for the control of the transmission device of FIG. 4; and
FIGS. 6 and 7 show two truth tables used by the flowchart of FIG. 5.